1. The Field of the Invention
The invention generally relates to the transmission of signals in communication systems. More specifically, the invention relates to systems and methods of mitigating the effects of dispersion in high frequency communication systems.
2. Description of the Related Art
In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through a fiber-optic cable using light emitting diodes or lasers. To send data on a fiber-optic cable, the data is typically converted from electronic data generated by computers to optical data that can be propagated onto the fiber-optic cable. This conversion is generally done using a laser or light emitting diode. When light is generated (or is at “high power”) a logical “1” is presented. The absence of light (or light at “low power”) represents a logical “0”. Thus an optical signal with sharp rising transitions and falling transitions representing logical “1” and “0” bits is transmitted onto the fiber-optic cable.
Much of the fiber-optic cable presently installed is multi-mode meaning that signals travel through different paths along the fiber-optic cable. For example, portions of a signal may travel directly down the fiber-optic cable while other portions of the signal “bounce” or are reflected back and forth along the fiber-optic cable. Further, for portions of the signal that are reflected back and forth, each portion may take a different path resulting in different velocities at which the portions of the signal travel through the fiber-optic cable. Thus, a single signal may take several paths resulting in dispersion of the signal as it travels along the fiber-optic cable. The dispersion of signals is more pronounced as the distances that the signals travel is increased or as the frequency at which the signals are transmitted increases. As the signals disperse, portions of adjacent bits may disperse into each other. For example, portions of a “1” or high power bit may spread into the time of the signal previously occupied by a “0” or low power bit. Similarly, the absence of power in a “0” or low power bit may cause a decrease in the power of an adjacent “1” or high power bit at various times within the time of the signal previously occupied by a “1” bit. This phenomenon is commonly referred to as intersymbol interference. These effects are often illustrated and studied using an open eye diagram such as the diagram shown in FIG. 2. As dispersion takes place, the high and low thresholds are blurred. The more blurring that takes place between the thresholds, the more “closed” an eye on the open eye diagram is. The more closed an eye is, the more difficult it is to interpret data bits.
Because of the dispersion, some of the data bits embedded in the electronic signals may be erroneously interpreted. While it is expected that some erroneous interpretations of data bits may occur, most communication standards specify a maximum number of erroneous interpretations that may occur. This is usually specified as the maximum bit error rate (BER). For example, the maximum BER in 10 Gigabit Ethernet systems is 10−12. To the extent that dispersion causes more errors than this, the effects of the dispersion must be mitigated.
While dispersion can be mitigated by installing single mode fiber, this solution is expensive to implement. Replacing the large amounts of fiber-optic cable that may be installed in a given location may be expensive and time consuming.
Alternatively, several different wavelengths can be transmitted over a single fiber. Each wavelength carries a signal at a lower bit rate to reduce errors. As such, the overall bit rate of a signal can be maintained while minimizing errors. This solution however would require multiple lasers to generate the different wavelengths, multiple photodiodes to receive the different wavelengths, and expensive multiplexing equipment to combine and separate the different wavelengths.
Similar problems exist in electronic (such as copper or other conductor based) transmission lines. For example, a small 7–10 inch transmission line formed on a printed circuit board may cause sufficient dispersion in an electronic signal being propagated at 10 Gigabits per second to effectively render the electronic signal unreadable.